CN115190870A

CN115190870A - Preform for speckle-free output fiber with structured silica sections, method of manufacturing such preform, and improved speckle-free output fiber

Info

Publication number: CN115190870A
Application number: CN202180016912.6A
Authority: CN
Inventors: 沃尔夫冈·纽伯格; 安德烈·格里申科
Original assignee: Fairfield Industries Inc
Current assignee: Fairfield Industries Inc
Priority date: 2020-02-25
Filing date: 2021-02-25
Publication date: 2022-10-14
Also published as: WO2021170748A1; JP2023514641A; US20230104707A1; MX2022010462A; EP4110736A1; CA3172773A1; BR112022016956A2

Abstract

Disclosed are fabrication of preforms and novel preforms that, after drawing, produce novel optical fibers and improved speckle-free output optical fibers. Useful fibers are made from the preforms described herein to provide a speckle-free smooth output with flat-top transmission of light from gaussian or few-mode sources. Unique fabrication of these improved preforms is also disclosed. The preform and the resulting fiber with core sizes varying from about 100 μm to over 1000 μm are based on a structured silica segment of the mode mixing region adjacent to the inner core, or in the case of a non-circular core, within the core. The plasma vapor deposition process was modified to achieve a structured zone in a well controlled manner. The structured section is made up of a plurality of pairs of layers, where thin down-doped layers alternate with much thicker layers of core material. The ratio of the thickness of the core layer to the thickness of the down-doped layer is about 3 to 25. The number of pairs of layers is typically between about 8 and 30 pairs of layers. The effective NA of the structured section depends on the details of the structured silica section and the respective lower doped layer. Both circular core examples and non-circular core examples are possible and discussed herein.

Description

Preform for speckle-free output fiber with structured silica sections, method of manufacturing such preform, and improved speckle-free output fiber

Introduction to

Technical Field

For many applications employing lasers and fiber lasers, it is preferable to have a speckle-free output at the distal output end of the fiber optic system rather than the typical gaussian profile available from laser sources. Such outputs are commonly referred to as Top-Hat (Top Hat) profiles or Flat Top (Flat Top) profiles. Generally, they require an efficient mode-mixing fiber section (section) to work well in most systems/applications. In other applications, a speckle-free output is required to provide speckle-free emission, for example for laser cleaning or spatially sensitive sensors. We disclose herein a manufacturing method and preform structure that can be drawn into a desired optical fiber having a speckle-free output on its output surface.

Background

Laser welding or bonding has become a very large business for a variety of applications. In many cases, it is critical to have a very clean and very smooth surface, down to atomic/molecular size, in order to obtain the full benefits of these processes. Laser cleaning has become an alternative method in preparing surfaces for precision and long-term welding, as well as for repainting and the like.

The need and consequent pursuit of speckle-free output fiber output has existed for some time, particularly after the use of laser and fiber laser sources has begun to be used in many applications. In addition to the need for improved ultra-clean surfaces for soldering and joining, the continued miniaturization of various opto-optical devices and the push for high-density power single-mode or few-mode sources has resulted in the need for speckle-free beams in laser processing systems for large and small components. Whether laser welding, laser cleaning, laser joining/sealing procedures, they may negatively impact the non-speckle-free beam. For example, creating ripples in the surface after cleaning with a laser having a speckle output achieves a regular bond, but is less dense, continuous, or defect free than what is ideal as laser bonding/welding can provide. Furthermore, in high power (CW or pulsed) sources, local power peaks can cause damage to the fiber itself. Avoiding this situation is highly desirable, especially in high power applications.

Optical fibers are commonly used to transport and distribute laser radiation to areas remote from the laser source. This may be advantageous to protect the source, provide a larger working beam, and/or more flexibly reach different surface targets. Generally, these benefits are due to the use of multimode optical fibers with large core counts, which have multiple modes of laser energy transmission.

Optical fibers are typically drawn from a preform. The cross-sectional structure of the preform determines the cross-sectional structure of the drawn optical fiber. Fabrication of the preform can be accomplished by a variety of processes, but one of the processes described herein is Plasma Outside Vapor Deposition (POVD). Here, the preform is constructed from a core rod by depositing successive layers of material to provide a cladding and a glass sheath. Sometimes after the cladding deposition is complete, the pure silica tube is fused to the preform to bring the outer diameter to the desired thickness. In this process, the core, cladding and outer pure silica are all coaxial with one another. Polygonal core preforms can be fabricated in a similar manner to circular cores, starting with a non-circular core and depositing cladding and jacket layers. Depending on the shape of the starting material preform, standard size optical fibers can be drawn from such preforms having a circular or non-circular core.

Laser bonding; there is a high demand for speckle-free, clean surfaces in electronics and high-tech miniaturization. Mode-mixing fibers do not always result in true flat-top output, creating or leaving ripples on the surface on a molecular scale that can affect the performance of equipment or high-tech applications (e.g., supersonic jets, high-value aircraft parts, space applications, etc.) that can actually damage/compromise the performance. The smaller the device, the more speckle-free the beam output needs to be over the output surface area of the delivery fiber. Otherwise, serious adverse effects may be produced on the treated workpiece surface. For example, the requirements for mode mixing required for an efficient cladding pumped fiber laser are relatively low compared to the laser processing requirements for micro to ultra micro electronics. An asymmetric core or a non-circular core alone is insufficient to produce the level of mixing required to produce a true top-hat output, which is a true speckle-free output over a wide range of input sources.

As a result, optical fibers with excellent mode mixing are required for many critical applications in laser cleaning, laser bonding, and laser welding to achieve speckle-free output. Ideally, such an optical fiber is simply drawn from a properly configured preform having all the necessary features of the final optical fiber in proportion to the draw ratio of the desired dimensions of the optical fiber.

In the prior art, several approaches have been taken to produce asymmetric core cross-sections by using asymmetric cores, non-circular cores, claddings containing destructive changes in refractive index by adding local sections of new material or air, and by changing the cross-section at different points along the long axis of the fiber by modifying the drawing parameters during the drawing process.

Disclosure and objects of the invention

Our technological advances in preform structure have increased the likelihood of better, more speckle-free performance of the optical fibers required for remote processes drawn from the preforms described herein, as well as new manufacturing techniques to achieve and provide the desired speckle-free output characteristics in optical fibers drawn from these preforms. The main objective is to design and produce an optical preform whose structure lends itself well (ideally) to allow the drawing of optical fibers of various sizes that are speckle-free output in the transmission of gaussian output sources or other sources that do not have speckle-free cross-sectional output.

Another object is to provide a preform for a speckle-free output fiber that can be drawn into a speckle-free output fiber using standard drawing processes, allowing the output of the drawing process to be lost and only maintaining additional costs in the preform fabrication process.

Another object is to provide a manufacturing process for a preform that can be used to make a speckle-free output fiber.

Other objects are to provide speckle-free output fibers for various laser processing of materials, including laser cleaning, laser processing, and laser welding. A circular core preform structure that meets these objectives is the objective of the current patent. Fabrication and processing of non-circular core preform structures that can also be successfully drawn into speckle-free output fibers is also an object of this patent application.

In summary, we describe a novel circular core; and a non-circular core preform for drawing a speckle-free output fiber of equivalent cross-section; and a method for producing the same. These preforms are designed to make better speckle-free output fibers. Different sized fibers (core sizes from 100pm to 1000pm and above) can effectively convert gaussian or low mode light source output to a speckle-free work surface output, such as a flat-top output. The new and improved speckle-free output fiber products made from these preforms are well suited for laser machining applications (including laser cleaning of surfaces and laser welding of critical surfaces) and other applications that benefit from top-hat type output.

Drawings

Fig. 1 shows the basic structure of an initial preform, which has a core and is surrounded by a structured silica layer, as can be seen in fig. 1A.

Fig. 2 shows the refractive index profile of a cross-section of the starting preform, with more detail in fig. 2A and 2B.

Fig. 3 shows an intermediate preform in which the shaded asymmetric regions have been ground away. The core was made asymmetric to the resulting preform as shown in fig. 4.

Fig. 4 shows the inner intermediate preform of fig. 3 surrounded by a reflective layer, ready for drawing a speckle-free output fiber.

Fig. 5 shows a preliminary stage of an intermediate preform for a non-circular core optical fibre, in relation to the initial preform in fig. 1.

Fig. 6 shows one of 2 preforms made from the intermediate preform shown in fig. 5, ready to be drawn into a non-circular core, speckle-free output optical fiber.

Fig. 7 basically shows a preform with a flat surface and two sets of cutting lines to create cores for 4 drawn preforms with non-circular cores.

Fig. 8 shows one of 4 preforms made from the initial preform of fig. 7, ready to be drawn into a non-circular core, speckle-free output fiber.

Fig. 9 shows a cross-sectional view of Plasma Outside Vapor Deposition (POVD).

The right side of fig. 10 is a near field image and plot of a circular core fiber of the present invention having a core diameter of 300 μm; and the near field image and curve for a standard round core fiber of the prior art with a core diameter of 300 μm is on the left.

The right side of fig. 11 is a near field image and plot of a round core fiber of the present invention having a core diameter of 600 μm; and the near field image and curve for a standard round core fiber of the prior art having a core diameter of 600 μm is shown on the left.

The right side of fig. 12 is a near field image and plot of a non-circular core fiber of the present invention having a core size of 100 μm x 100 μm.

Detailed Description

In the following description, for the features shown in fig. 1 to 8, reference numerals with the same last two digits are similar items, e.g. 101, 201, 301, 401, etc. are pure silica cores, which comprise an inner core, and each of 103, 203, 303, 403, 503, etc. is a structured silica mode-mixed region composed of a deposit of under-doped silica and pure silica, which in each case surrounds the inner core in the figures, as described below. Although most of the depositions described herein use a plasma-out-of-body vapor deposition (POVD) process, a Plasma Chemical Vapor Deposition (PCVD) process can also be used in the different deposition steps described herein if desired for general reasons. Plasma deposition as referred to herein may refer to any process, if not specifically stated. Pure silica core rod 101 has been placed in a POVD chamber to add a series of layers that alternate between lower doped layers 123 and pure silica layers 121, resulting in the structured section 103 seen in fig. 1. The difference between the diameter of the pure silica core 102 and the diameter of the structured silica section 104 defines the total thickness of the mode-mixed structured silica section 103. Within the section 103, there are a plurality of hierarchical pairs 120, which may be different for different situations, typically in the range of 8 to 30 pairs. Within each segment pair 120, the pure silicon dioxide layer 121 is typically much thicker than the lower doped silicon dioxide layer 123. The ratio of the two thicknesses typically ranges from about 1 to 20. This is summarized in fig. 1 and 1A. Particularly useful ranges for these two parameters are a thickness ratio of 7-13 in the pair of layers and a number of pairs of layers of 12-20.

Of course, starting from a suitably sized silica core, the

inner core

101, 201 may be made of a thinner silica rod on which, in some cases, pure silica is deposited by additional plasma deposition of pure silica to achieve the desired core diameter.

Fig. 2 shows the Refractive Index (RI) distribution of the preform 100 in a cross-sectional view. Fig. 2A and 2B show how RI varies in cross section. These lines represent the refractive index dip of the down-doped silica layer between the refractive indices of the core materials. The steepness of the RI change indicates a steep change in the material during deposition and the speckle-free bottom determines the speckle-free of dopant levels in each lower doped layer. In a series of examples, Δ n =5 × 10 ^-3 。

After taking the preliminary form of fig. 1, it is deposited with additional pure silica 305 to form a preform having a diameter 325, as shown in fig. 3. In the next step, a preform having an asymmetric inner core is fabricated by grinding away portion 307 of the original preform diameter 325, preferably to one side of the preform, such that the new preform shape has a structured silica segment 303 surrounding inner core 301 that is offset from the center of the new ground shape.

Fig. 3 depicts an asymmetric removal of outer material 307, wherein inner core 301 is off-center within outer core 305. Core 301 is concentrically surrounded by a region 303 of structured silica, the diameter 302 of the inner core and the diameter 304 of the structured silica defining the total thickness of the region of structured silica.

FIG. 4 shows a cross-sectional view of a finished preform ready for drawing into an optical fiber having a speckle-free output. Inner core 401 is concentrically surrounded by structured silica 403, the thickness of which is defined by the difference in the diameters of structured silica 404 and inner core 402. The outermost core 405 is surrounded by a reflective layer 409, such as a POVD/PCVD deposited down-doped silicon dioxide. Note that the center of inner core 401 is offset in outer core 405 by difference 411. In one example, 411 is 4mm.

We can also first explain the cross section of an optical fiber drawn from a preform as described above using fig. 4. In this case, the reflective layer 409 may be applied when drawing the fiber and may therefore be selected from silicone, hard plastic cladding, other polymer cladding materials. The reflective layer 409 of the speckle-free output fiber may also be composite, i.e., the fiber may be drawn into a preform with a reflective layer added during the drawing process.

Another point should be supplemented. Although silica glass optical fibers are very strong when drawn, over time the glass surface is susceptible to damage from various application conditions that may damage the outermost glass layer. Thus, it is well known that, typically, optical fibers used in open environments, as found in most industrial or medical applications, typically have one or more protective outer coatings (jackets) not described herein. These topcoats are typically applied during the stretching process, but they may be applied in further downstream processing.

Fig. 5-8 illustrate aspects of producing preforms and optical fibers having non-circular cores for speckle-free output. First, the initial preform shown in FIG. 1 is enlarged with additional core material to produce a larger preform having an inner core 501, a region of structured silica 503, and a second core surrounding the region of structured silica, which has a diameter 525. The secondary core may be prepared entirely by a plasma deposition process, or alternatively by sleeving a pure silica tube (the inside dimensions of which closely match the diameter of the original preform) and then bonding the two into a larger preform without bubbles and with the desired diameter 525. The larger preform is ground to remove material 507 until it reaches its width 515, having a particular height relative to its width. The larger preform is ground such that a portion of the second core material remains on all of the structured silica regions 503. For most examples, the inner core 501 and the outer (second) core 505 are both pure silica materials. Cutting the ground preform along cutting line 513 to generate two non-circular cores of two new preforms; each of which can be drawn into a speckle-free output fiber.

In FIG. 6, each composite core of FIG. 5 is set in a plasma deposition apparatus and after rounding the corners 619 thereof, a reflective coating 609 is deposited on the composite non-circular core. The

core materials

601 and 605 are generally the same and the core has a semicircular region 603 of structured silica therein. Its width 615 is as shown. In this particular example, the width and height are substantially equal in length, and the shape of the non-circular core is square. Other shapes are also possible, such as rectangular, triangular, trapezoidal, hexagonal, octagonal, etc.

The optical fiber drawn from the preform will have an equivalent cross-section, the actual dimensions of which are proportional to the preform. In one preform example, the diameter of inner core 501 is 15mm. The diameter of the structured silica 503 was 17mm, so that the thickness of the structured

silica

503, 603 was 2mm. The width and height are equal and 18.5mm. And the preform diameter 525 is 51mm.

Fig. 7 and 8 depict dividing a ground initial preform having a non-circular core into 4 equivalent square cores and generating 4 new preforms having the cross-sections shown in fig. 8. Thus, in fig. 7, inner core 701 is surrounded by structured silica 703 and then by additional core material 705. The initial preform has a diameter 725. After the initial deposition, the preform is ground into a rectangular composite core having side dimensions 735 by removing material 707 and then cutting the resulting rectangular core into 4 non-circular core pieces along cut lines 713. The corners of these pieces are then rounded and a reflective layer 709 deposited to produce 4 similar preforms, as shown in figure 8. As previously described, inner core 701 and second core 705 are typically the same material, most likely pure silica.

As shown in fig. 8, the final preform has an arcuate shaped structured silica 803 sandwiched between core material 801 and core material 805 within a square core, which for this example has a rounded corner 819, with reflective material 809 surrounding the core being deposited or otherwise added to form the final preform. The width of the non-circular core 835 is equal to the height of the core, since the core is square in this example. Other possible shapes for the non-circular core are mentioned above. The relative area in the drawn fiber will be proportional to the area of the preform shown in fig. 8, since the shape of the fiber cross-section is the same as the shape of the preform.

In one example, the diameter of the inner core of pure silica 701 is 15mm and the diameter of the surrounding structured silica 703 is 17mm, such that the thickness of the structured

silica

703, 803 is 2mm. Diameter 725 is 51mm. Each of the 4 non-circular cores has a side dimension 735, 835 of 18.5mm x 18.5mm.

A typical POVD setup is shown in fig. 9, where 901 is the screening bin; 902 is a substrate rod; 903 is a glass processing lathe; 904 is a plasma torch; and 905 is a handle attached to the base rod 902. In many examples, the plasma torch 904 was operated at 5.28MHz and 50kW power level. Note that plasma vapor deposition, i.e., POVD or PCVD, was performed in different cases.

Within the structured silica segments and the reflective coating, a wide range of materials can be used as core materials. Pure silica is typically selected as the core material and used for the sleeve, but up-doped Si, such as germanium-doped silicon (Ge-Si) or graded index silica-based cores may be used. The reflective layer is most commonly a fluorosilicate, but other lower index silicas, such as borosilicates, may also be used. In the reflective/cladding type coating, the coating applied after fiber drawing includes fluoroacrylate and silicone plastic materials. The choice of core material will influence the viable choice of materials for the pairs of layers of the structured silica segment. For example, using pure silica as the core material, the lower doped (lower RI) silica will be the first of the pair of layers, such as Fluoro-silica (Fluoro-silica) with a selected F dopant level, and the second higher RI layer may be selected from: pure silica, or down-doped fluorine silica, or up-doped silica (such as Ge-Si), or similar materials, as long as the total refractive index of the structured silica segment is lower than the desired core refractive index of the optical fiber. Some special effects may occur where one or more of any of the pairs of layers becomes up-doped silica, as long as the refractive index of the structured silica segments remains below the core refractive index.

The preferred combination, ratio of thicknesses within the paired layers, and number of paired layers are numerous, depending on the intended application, available preform equipment and materials, and core requirements. Some more useful ranges of the number of layers in a pair of layers versus the thickness therebetween have been described previously.

Additionally, to make fiber lasers or amplifiers, the rare earth doped innermost core can be incorporated in the structure of silica or other core material, incorporated in the preform and thus in the drawn fiber, and added with a structure of structured silica or the like. Alternatively, a tubular preform can be fabricated and then sleeved over a rare earth core or clad rare earth core rod.

Fig. 10-12 show some representative results for optical fibers made from preforms having a structured silica segment contained within the core thereof. In particular, each figure has a near field image on the right and below is a corresponding output plot of three sample fibers with a 300 μm circular core, a 600 μm circular core, and a non-circular 100 μm x 100 μm square core. For comparison, in fig. 10 and 11, the corresponding near field images and curves for standard 300 μm and 600 μm core fibers are shown in the left half, respectively.

At the time of filing, a core optical fiber of 300 μm core, 600 μm core or more will be a preferred example of the present invention. For non-circular core fibers, the preferred non-circular core pattern would be a square or rectangular core with a half-circular arc of structured silica or a quarter-circular arc of structured silica.

Another potentially useful configuration would have a thin upper doped layer immediately before or after the structured silicon dioxide segment described above; or a thin upper doped layer before and after the previously described structured silicon dioxide segment herein. The thickness of the upper doped layer should be as thin as or thinner than the lower RI layer of the pair of layers.

Claims

1. A preform for a speckle-free output optical fiber drawn from the preform, the preform having a cross-sectional structure comprising:

a circular inner core having a refractive index or refractive index profile, the circular inner core being surrounded by structured circular regions having an average refractive index lower than the average refractive index of the inner core;

which can be drawn into speckle-free optical fiber using standard fiber drawing techniques.

2. The preform of claim 1, wherein the preform,

wherein the structured circular region has a number of pairs of layers starting from the inner core with a first layer having a lower Refractive Index (RI) than the core material, followed by a next layer having a higher RI than the first layer material, and wherein each layer has a thickness.

3. The preform of claim 2, wherein the preform is,

wherein the lower RI layer is a lower doped layer and the next layer is a core layer or an upper doped layer.

4. The preform of claim 2 or 3,

wherein, in each of the pair of layers, a ratio of a thickness of the core layer to a thickness of the lower doped layer is about 1 to about 20.

5. The preform of claim 2 or 3,

wherein the number of pairs of layers is from about 8 to about 30.

6. The preform according to any one of claims 1 to 5,

wherein a tube of pure silica is wrapped thereover without creating any gaps or any bubbles at the interface between the inner surface of the tube and the second cladding to form a drawn preform for the speckle-free output optical fiber.

7. An optical fiber drawn from a preform according to any one of claims 1 to 6, the cross-section of said optical fiber being proportional to the cross-section of said preform and the output/transmission of even a high power, low mode photon source of said optical fiber having reduced speckle.

8. The optical fiber according to claim 7, wherein said optical fiber,

wherein the structured circular region is as defined in any one of claims 2 to 5.

9. Method for manufacturing a preform according to any of claims 1 to 6, wherein plasma vapour deposition is used to produce a section of the cross-section of the preform and its layers as described in the claims.

10. A preform from which a speckle-free output optical fiber can be drawn, a cross-sectional structure of the preform comprising:

a composite non-circular core surrounded by a reflective cladding-type material;

wherein the composite non-circular core further comprises:

a section of polygonal core material having a refractive index; and

an arc of a circular region of structured silica within a section of the polygonal core, the average refractive index of the arc of the circular region of structured silica being lower than the average refractive index of the core material;

the refractive index of the reflective cladding type material is lower than the refractive index of the core material; and

which can be drawn into a speckle-free output fiber using standard fiber drawing techniques.

11. The preform of claim 10, wherein the preform is,

wherein the structured circular region has pairs of layers starting from the inner core, the pairs of layers comprising a lower doped layer and a subsequent core layer; and wherein each layer has a thickness.

12. The preform of claim 10 or 11,

13. The preform of claim 10 or 11,

wherein the number of pairs of layers is from about 8 to about 30.

14. Preform according to any one of claims 10 to 13,

wherein the polygon is selected from the group of a triangle, a rectangle, a pentagon, a hexagon, a heptagon, an octagon, a decagon, and a dodecagon.

15. Preform according to any one of claims 10 to 14,

wherein, for a 4-sided polygon, the section of polygonal core is a rectangular/square core; or for all other polygonal shapes, a generally pie-shaped core.

16. The preform of claim 15, wherein the preform is,

wherein the arc segments of the structured silica circular region are different for different polygonal core shapes, typically a portion of the circular region divided by the number of sides in the polygon.

17. The preform of claim 16, wherein the preform is,

wherein the arc segments of the structured silica circular region within a rectangular/square core have a semi-circular shape when the precursor initial rectangular core is cut only once through its long side prior to deposition of the reflective cladding around the non-circular core.

18. An optical fiber drawn from a preform according to any one of claims 10 to 17, the cross-section of said optical fiber being proportional to the cross-section of said preform, and the output/transmission of even a high power, low mode photon source of said optical fiber being speckle-free output.

19. The optical fiber according to claim 18, wherein,

wherein the structured circular region is as defined in any one of claims 10 to 17.

20. Method for manufacturing a preform according to any of claims 10 to 17, wherein plasma vapour deposition is used to produce the section of the cross-section of the preform and its layers as described in the claims.

21. The preform of any one of claims 1 to 6 or 10 to 17,

the preform also includes an innermost core of a high refractive index, rare earth doped material, such that the fiber can be used as a fiber laser/amplifier or sensing medium when drawn.

22. The optical fiber of any one of claims 7 to 8 or 18 to 19,

wherein the relevant preform has an innermost core of rare earth doped material, such that the fiber can act as a fiber laser/amplifier or for sensing purposes.